The present invention relates to a surface roughening method, and more particularly to a method of providing an uneven surface for glass and metal plates that may be used as substrates for solar cells.
The concept of "textured cells" was introduced by Wronski et al, with a view to improving photoelectric conversion efficiency of thin-film solar cells using amorphous silicon or other semiconductor materials. According to Wronski's theory, the incident sunlight is caused to deflect by some means and the resulting confinement of the transmitted light is used to increase the amount of photocurrent (see Unexamined Published Japanese Patent Application No. 59-61873, corresponding to U.S. patent application Ser. No. 424,137/1982 now U.S. Pat. No. 4,532,537 issued July 30, 1985).
Exemplary solar cells utilizing the confinement of incident light are shown in FIG. 2. The cell shown in FIG. 2a consists of a glass plate 11, one major surface of which is textured and overlaid with a transparent electrode 12, an amorphous silicon (a-Si) layer 13 having a p-i-n junction and a back metal electrode 14, in the order written. The sunlight 17, falling perpendicularly on the glass plate 11, deflects at the interface betweend the substrate and the transparent electrodes 12, and the light having longer wavelengths that has transmitted through the amorphous silicon layer 13 impinges on the interface between this amorphous silicon layer and the back electrode 14 at a large angle of incidence, and resulting enhanced reflectance at this interface allows a greater amount of light to be absorbed by the amorphous silicon layer.
The cell illustrated in FIG. 2b uses a textured metal substrate 15, rather than the textured glass plate 11 and, on this metal substrate, an amorphous silicon layer having a p-i-n junction, a transparent electrode 12 and a collector electrode 16 are deposited, in the order written. In this case, the incident sunlight is effectively confined in the amorphous silicon layer 13 by entirely the same mechanism as explained for FIG. 2a. The cell shown in FIG. 2c has the same construction as that of the cell in FIG. 2a, except that the glass plate 11 has no textured surface and, instead, the transparent electrode 12 is provided with a textured surface.
Conventionally, the surfaces of glass plates, metal plates, etc., are roughened either by using abrasives or by using more efficient (faster) chemical etching techniques, providing natural-looking uneven surfaces. However, it is very difficult to implement either method while ensuring precise control over the size and height of the projections on the roughened surface.